Sir Richard Branson's Virgin Galactic has been urged to "give up" on its space bid, following claims that safety warnings were not heeded prior to its $500 million vehicle exploding in mid-air.

As investigators began the painstaking process of collecting debris scattered across California's Mojave desert, one expert said Virgin should abandon its already much-delayed project.

"They should stop, give up. Go away and do something they might be good at like selling mobile phones. They should stay out of the space business," said Carolynne Campbell-Knight, lead expert on rocket propulsion for the Netherlands-based International Association for the Advancement of Space Safety (IAASS).

Ms Campbell said she sent multiple warnings to Virgin since 2009 about the dangers of using nitrous oxide in its propulsion system because it could "go bang in a very unpredictable way".

Virgin Galactic said it was cooperating with an independent investigation and "during that time we are not permitted to comment on the accident".

However, George Whitesides, Virgin Galactic chief executive, dismissed claims that safety warnings had been ignored, saying he was not aware of any official warnings from the IAASS. He told the Financial Times: "In the space community you will be able to find people who have favourite technologies of different types. One group will say their type of technology is better than another."

In 2007, three engineers died in an explosion during testing of the spacecraft's rocket engine on the ground.

SpaceShipTwo had been flying for the first time with a new solid plastic-type propellant rather than the rubber-based fuel used previously. It was ignited by nitrous oxide.

One former Nasa astronaut speculated there could have been a structural failure and rapid loss of pressure in the cockpit, followed by the plane breaking up, rather than a sudden explosion.

Chris Hart, acting chairman of the National Transportation Safety Board, said his team found three types of tank on the desert floor containing the plastic fuel, nitrous oxide and methane.

He said the investigation could take 12 months. Sir Richard has called early speculation about the cause "slightly irresponsible".

A witness at the site of the crash suggested that the parachute of Mike Alsbury, 39, the test pilot who died, may have partially deployed.

Doug Messier said the pilot landed, still strapped into his seat, at the side of a road. "I could see a small part of the parachute had deployed but he didn't have a chance to get out or deploy. It's very sad," he said.

The second pilot Pete Siebold, 43, parachuted to safety. SpaceShipTwo did not have ejection seats, but did have an escape hatch.

Virgin has already been building another spacecraft for several years, and it is expected to be completed next year.